Vega encircled by asteroid belt

Kate Taylor, 9th January 2013

Astronomers believe they've found a large asteroid belt around Vega, the second brightest star in northern night skies.

If so, it's the second such belt to have been found outside our own solar system, with the other encircling Fomalhaut. Both stars appear to have inner, warm belts and outer, cool belts separated by a gap - similar to the asteroid and Kuiper belts in our own solar system.

As to what's maintaining the gap between the warm and cool belts around the stars, the results strongly suggest it's multiple planets. Our own asteroid belt, which lies between Mars and Jupiter, is maintained by the gravity of the terrestrial planets and the giant planets, with the outer Kuiper belt shaped by the giant planets.

"Our findings echo recent results showing multiple-planet systems are common beyond our sun," says Kate Su, an astronomer at the Steward Observatory at the University of Arizona.

Both Vega and Fomalhaut are about twice the mass of our sun and burn a hotter, bluer color in visible light. Both are about 25 light-years away and are thought to be around 400 million years old - although Vega could be more like 600 million. Fomalhaut has a single candidate planet orbiting it, Fomalhaut b, which orbits at the inner edge of its cometary belt.

To identify the asteroid belt, the Herschel and Spitzer telescopes detected infrared light emitted by warm and cold dust in separate bands, replenished by comets and chunks of rock. The inner belt can't be seen in visible light because the glare of the star outshines it.

Both the inner and outer belts contain far more material than our own asteroid and Kuiper belts. This is because the star systems are far younger than our own, and the system likely formed from an initially more massive cloud of gas and dust.

The gap between the inner and outer debris belts for both Vega and Fomalhaut also corresponds to the distance between our sun's asteroid and Kuiper belts, with the outer belt 10 times farther away from its host star than the inner belt.

The large gap between the two indicates that there are several undetected planets, Jupiter-sized or smaller, creating a dust-free zone between the two belts by sweeping up debris.

"Overall, the large gap between the warm and the cold belts is a signpost that points to multiple planets likely orbiting around Vega and Fomalhaut," says Su.